A fuel cell which generates power by utilizing an electrochemical reaction between hydrogen rich gas and oxygen-containing gas such as air has been heretofore known. In the fuel cell, usually, air is used as the oxygen-containing gas and reformed gas is used as hydrogen-containing gas, the reformed gas being obtained by a reforming reaction which generates hydrogen and carbon dioxide by use of a hydrocarbon fuel (for example, gasoline), water and oxygen. A fuel reforming device which performs the reforming reaction includes a plurality of components such as: a fuel reformer using any one of or both of a steam reaction and a partial oxidation reaction; a shift reactor which reduces a concentration of carbon monoxide in the reformed gas by a shift reaction; a selective oxidation reactor which reduces the concentration of carbon monoxide in the reformed gas by a selective oxidation reaction; and a heat exchanger which is provided between the reactors and maintains a proper temperature of reformed gas.
In order to adopt a fuel cell system in a vehicle drive power source, shortening of start-up time is one of problems to be solved. Particularly, it is a problem how quickly the respective components of the fuel reforming device can be heated to a temperature suitable for reactions to generate hydrogen rich gas.
As a conventional method for heating the fuel reforming device, there is a technology of heating the respective components of the fuel reforming device by supplying a fuel and air to a combustor in start-up for combustion thereof and distributing obtained combustion gas within the fuel reforming device. However, when a hydrocarbon fuel that is a liquid at room temperature, such as gasoline, is used as the fuel, there occurs diffusion combustion if the fuel is combusted by being injected into air in the combustor. Thus, there was a problem that high-concentration nitrogen oxide (NOx) is generated in the combustion gas. For a problem similar to the above, as a technology of reducing emissions in general combustors, there is a technology of performing premixed lean combustion by pre-vaporizing a liquid fuel, premixing the liquid fuel with air at a rate of excess air and supplying the mixture to the combustor.
For example, there has been known a technology in which combustion gas obtained by performing combustion at a mixing ratio close to a theoretical mixing ratio (theoretical air-fuel ratio) in a sub-combustion part is mixed with air, high-temperature gas, of which temperature is lowered below a ignition temperature of a liquid fuel, is generated, the high-temperature gas is mixed with the liquid fuel to be pre-vaporized and the mixture is supplied to a main combustion part in a subsequent stage in order to perform premixed lean combustion (see Japanese Patent Application Laid-Open No. H4-177011).